The present invention relates to a drive circuit for a firing element of an occupant protection system of a motor vehicle.
Occupant protection systems of a motor vehicle are airbag or seatbelt pretensioners, by way of example. Protection systems of this type are triggered by a firing element, for example a pyrotechnic firing pellet, which initiates further operations for opening an airbag or for tensioning a seat belt. The firing pellet is usually activated by a predetermined activation current/firing current, for example 1 A to 3 A, being applied to it for a predetermined activation duration, for example 0.5 ms to 5 ms, by a drive circuit.
For safety reasons, the drive circuit must have at least two switching elements which are connected into the load circuit of the firing element in the drive circuit and which isolate the firing element in each case from supply potentials. In this case, both must be driven in conducting fashion for activation of the firing element. Such a drive circuit for a firing element of an occupant protection system is disclosed for example in the publications DE 196 17 250 C1, DE 199 34 559 C1, US 2001/0006309 A1 or DE 102 55 115 B3.
Referring to cited DE 102 55 115 B3, known drive circuits for such firing elements are constructed such that the two semiconductor components or semiconductor switches with which the firing element is connected in series are integrated in a common semiconductor body or semiconductor chip. This integration of the two semiconductor switches on a semiconductor chip may have the effect that in the event of a serious fault on the chip, for example triggered by an uncontrolled influence externally, an uncontrolled activation (IAD=inadvertent deployment) of the occupant protection system may occur.
A complete redundancy of the system, in the sense that when there is a fault in one semiconductor switching element, the other semiconductor switching element reliably prevents faulty triggering of the firing element, is not completely provided when both switching elements are integrated on a chip. Faults on the semiconductor chip which may lead for example to one semiconductor switch being switched on unintentionally may in many cases also lead to the second semiconductor switch being switched on unintentionally.
In order to avoid this disadvantage, DE 101 09 620 C1 discloses providing two integrated drive circuits of identical type each having two semiconductor switching elements and connecting up these two drive circuits in cross-coupled fashion. In this case, a firing element is connected in each case between a semiconductor switching element of one drive circuit and a semiconductor switching element of the other drive circuit. In the case of a cross-coupled circuit, therefore, provision is made for using semiconductor switches of different drive circuits for driving a firing element. A disadvantage of such an arrangement is the comparatively complex wiring, particularly in the case of multichannel systems in which more than two firing pellets are intended to be driven.
WO 97/32757 discloses a drive circuit for a firing element of an occupant protection system in which two semiconductor switches provided for driving a firing element are realized as discrete components, that is to say as components which are integrated in different semiconductor bodies.
The applicant's hitherto unpublished application DE 103 44 841.1 describes a drive circuit for a firing element of an occupant protection system, in which two semiconductor switches provided for driving a firing element are integrated in different semiconductor bodies which are applied to a common carrier and are arranged in a common chip housing.
In the case of such an arrangement of the two semiconductor switching elements in a common chip housing, a fault in one semiconductor switching element, for example thermal overheating of a component on account of a short circuit, may lead to an undesirable activation of the other semiconductor switching element and thus to an undesirable firing of the firing element.
Therefore, it would be advantageous to provide a drive circuit for a firing element of an occupant protection system which has at least two semiconductor switching elements which are integrated in separate semiconductor bodies and are arranged in a common chip housing, and which has a reduced susceptibility to disturbances.